The present invention relates generally to sensor devices and methods for monitoring conditions in a surrounding medium, and is particularly concerned with fiber-optic sensor devices.
It is often important to be able to monitor various properties in gaseous and liquid media. Some properties which can be critical are humidity, contaminant levels, pollutant levels, battery solution composition, and so on.
Current means of measuring air humidity involve either a special polymer sensor head whose capacitance is dependent on the amount of moisture it absorbs, or a dew point meter. Both have adequate accuracy, bit are slow in response time and expensive to manufacture.
Some proposals have been made in the past for fiber-optic sensor devices which detect changes in the refractive index of a surrounding medium, for various applications. A conventional optical fiber has a light transmitting, optical fiber core of glass, an outer cladding of different refractive index from the core to prevent optical loss from the core, and an outer protective layer, usually of plastic. In U.S. Pat. Nos. 4,851,817 and 5,005,005 of Brossia et al., an optical fiber has portions of both the outer protective layer and the cladding layer removed, exposing the core. The exposed core is provided with striations by abrading or sanding it with a piece of sandpaper or the like. The sensor surface irregularities cause light to refract out of the fiber and into the surrounding medium, with the amount of light lost being dependent on the refractive index of the surrounding medium. A photodetector senses the amount of light transmitted along the fiber past the sensor portion. Changes in the amount of light transmitted provide an indication of changes in the surrounding medium. Brossia describes possible uses for this sensor device in providing an indication of icing conditions on aircraft, and also for soil moisture detection, detection of leakage from underground storage tanks, and for fluid level detection.
One problem with this sensor device is that the optical loss through a length of bare fiber core is very high. In fact, 90% or more of the light may be lost under such conditions. Thus, this device is only capable of detecting gross changes in the refractive index of a surrounding medium. This sensor device will not be sensitive enough to detect small changes in air humidity, for example.